This invention relates to a method for joining foils by means of mechanical coating at a speed of preferably more than 50 m/min using an adhesive coating mass and subsequent lamination of the foils.
It is known that foils, consisting of the most varied materials, such as cellulose derivatives, polyolefins, polyesters, polyamides, metals such as tin or aluminum, which may also be subject to surface treatment by flaming or dark discharge or which, to improve the physical properties, can be coated with synthetic substances, for example, polyvinylidenechloride, can be joined together by means of glue systems in order to combine the properties of the various foils into one complex. It can be the objective of such an effort to achieve special decorative effects or to bring about technical effects, such as protection for an imprint, production of highly heat resistant complexes, stopping, vapor diffusion, heat-sealability, reliable avoidance of porosity, and resistance against aggressive substances.
Particularly appreciated among the adhesive systems employed for such coatings are cross-linked bonding agent systems involving a reaction of isocyanate groups with hydroxy-functional compounds.
According to the state of the art, this adhesive system as a rule consists of the following:
(A) Solutions of hydroxyl-group-containing polyesters, polyether or simultaneously ester-and ether-group-containing hydroxy-functional condensation products which, shortly before being used for coating, are mixed with a solution of polyfunctional isocyanates and which then form cross-linkages while forming urethane-or polyurea compounds.
(B) Solutions of free-isocyanate-group-carrying preadduct from hydroxy-functional polyesters, polyethers, or mixed polyether-esters, which, following the addition of polyols or simply due to the action of water from the environment (air humidity or adsorbed moisture from boundary surfaces) cross-link while forming higher-molecular polymers via urethane-and/or polyurea compounds.
It is characteristic of such coating adhesive systems, which are used on a large scale today, that they are applied upon the foils to be joined from a relatively low-visosity, low-solid solution, containing large quantities of solvent. The application is mostly via roller application mechanisms and in very small layer thickness at ambient temperature. It is also a characteristic that, prior to joining the coated foils in the laminating plant into layered systems having good initial and aging adhesion values, the solvents, which account for 60-90% by weight of the wet application and which are mostly quite valuable, volatile, combustible and partly physiologically objectionable, are completely removed to the greatest possible extent in effective drying stages and are disposed of without harming the environment.
Here are some particular disadvantages deriving from the laminating adhesive systems (a) and (b) used in the production of high-grade foil coatings based on the state of the art:
1. To generate sufficient initial adhesion, higher-molecular bonding agent systems are required which can be applied only from solutions of relatively low concentration at room temperature and in a sufficiently thin, well-wetting and uniform manner.
2. The solvents (ketones, esters, aromatic substances) are mostly very expensive and are often physiologically objectionable; considering the presently customary very high coating speeds, it is very difficult to remove them, that is to say, this can be done only with a tremendous energy expenditure, with plants having long evaporation assembly lines, large suctioning systems and room ventilation systems, exhaust air combustion (catalytic or thermal), large quantities of fresh air, and therefore high energy costs.
3. The solvent-containing adhesive systems are undesirable in terms of storage and transportation because of the level of hazard involved.
4. The transportation and packaging volumes are disproportionately high compared to the thin layers of adhesive in the laminates to be produced.
5. The residual solvents in the film, which are difficult to remove, for example, in the case of packaging complexes, endanger the materials to be packaged because they exert influence in the form of taste and odor.
6. In the case of NCO-group-containing adhesive systems, the solvents are particularly expensive because they must be completely free of water prior to use.
7. Two-component systems are particularly hard to handle in the plant and increase the danger of mistakes during the combination of components in the plant.
8. The low-visosity isocyanate component solution entails dangers in handling, especially for inexperienced personnel, such as this would be unavoidable during transportation.
The effort to avoid these disadvantages--by trying to develop cross-linking two-component lamination adhesive which would contain as little solvent as possible, and which involve reaction of isocyanate groups with alcoholic hydroxyl groups and possibly water--has so far failed because it was considered necessary, in order to achieve good adhesion (especially good initial adhesion), that the laminating adhesive bonding agents, contained in the solution, possess a high "average molar weight" and high viscosity (i.e., firm consistency). These properties however rule out any application in a form devoid of solvents.
It is furthermore known that one can apply thermoplastic synthetics in solvent-free form, so-called hotmelts, that is to say, waxes or copolymers from the melt, upon the foils and that one can use them to laminate foils. Such composite structures in particular entail the disadvantage that the coatings must be applied in a comparatively great thickness, that the coatings are not transparent, and that the composite systems have no heat resistance because of the thermoplastic nature of the coating. Moreover, these hotmelts are sensitive to aggressive filler materials.
It is known that one can avoid the disadvantages of thermoplastic hotmelts by means of solvent-free adhesive systems consisting of several components (U.S. Pat. No. 3,840,419). The desired small application quantities of 0.5-5mu are achieved through pre-dosing, by dosing elements known per se and by further stretching the medium to be applied by at least 1:10 by means of differing roller speeds in the application elements or through different speeds of the application roller and the substrate to be coated. A further compelling prerequisite in this method is that the obviously very fast setting adhesive must always be transferred completely from one roller to the next one, respectively, from the application roller to the film to be coated, in order to avoid premature hardening on the rollers. For this purpose, the adhesive is charged into the dosing device by means of a special feeder device and only in that quantity in which it is finally transferred to the film to be coated.